Soldering methods used for electronic parts include soldering with a soldering iron, flow soldering, reflow soldering, and the like.
Soldering with a soldering iron is a soldering method in which a flux cored wire solder is placed against a portion to be soldered and the solder wire is heated and melted with a soldering iron. Soldering with a soldering iron carries out soldering at one portion to be soldered at a time, so it has problems with respect to productivity and it is not suitable for mass production.
In the flow soldering method, the surface to be soldered of a printed circuit board is contacted with molten solder to carry out soldering. It has excellent productivity in that an entire printed circuit board can be soldered in a single operation. However, in the flow soldering method, with electronic parts having a narrow pitch, bridges may be formed in which solder straddles the parts and adheres thereto, and molten solder directly contacts electronic parts, and when the electronic parts have low resistance to heat, the electronic parts sometimes underwent thermal damage and had a deterioration in function. In addition, if connecting members such as connectors are mounted on the surface to be soldered of a printed circuit board, there is the problem that molten solder penetrates into holes in the connectors and they can no longer be used.
The reflow soldering method is a method in which a solder paste comprising solder powder and a flux is applied just to necessary locations on a printed circuit board by printing or dispensing, electronic parts are mounted on the portions where the solder paste was applied, and then the solder paste is melted in a heating apparatus such as a reflow furnace to solder the electronic parts to the printed circuit board. In the reflow method, not only is it possible to perform soldering of many locations in one operation, but there is no occurrence of bridging even with electronic parts having a narrow pitch, and solder does not adhere to unnecessary locations, so soldering can be carried out with excellent productivity and reliability.
In recent years, miniaturization of electronic equipment is progressing, and electronic parts mounted on printed circuit boards are being reduced in size. Therefore, printed circuit boards which are too fine to be soldered by flow soldering are increasing in number, and there is an increasing shift from flow soldering, which was conventionally used, towards reflow soldering.
Solder paste which has been used in the reflow soldering method from in the past has been one in which solder powder was a Pb—Sn alloy. This Pb—Sn alloy has a melting point of 183° C. with a eutectic composition (Pb—63Sn), it has little thermal effect even on electronic parts having low heat resistance, and it has excellent solderability, so it has the advantages that it produces few soldering defects such as unsoldered portions or dewetting. When electronic equipment which was soldered using a solder paste using this Pb—Sn alloy becomes old or malfunctions, it is discarded without being upgraded or repaired. When printed circuit boards are discarded, they have been disposed of by burial instead of incineration. Disposal is carried out by burial because solder is metallically adhered to the copper foil of printed circuit boards, and it is not possible to separate the copper foil and the solder and reuse them. If printed circuit boards which are disposed of by burial are contacted by acid rain, Pb in the solder is dissolved out and pollutes underground water. If humans or livestock drink underground water containing Pb for long periods of time, they may suffer lead poisoning. Therefore, there is a strong demand from the electronic equipment industry for so-called lead-free solders which do not contain Pb.
Lead-free solder has Sn as a main component, and currently-used lead-free solders are binary alloys such as Sn—3.5Ag (melting point of 221° C.), Sn—0.7Cu (melting point of 227° C.), Sn—9Zn (melting point of 199° C.), and Sn—58Bi (melting point of 139° C.), as well as those to which a third element such as Ag, Cu, Zn, Al, Bi, In, Sb, Ni, Cr, Co, Fe, Mn, P, Ge, and Ga is suitably added. In the present invention, “based” means an alloy itself or an alloy based on a binary alloy to which at least one additional element is added. For example, a Sn—Zn based alloy means a Sn—Zn alloy itself or an alloy having at least one of the above-mentioned additional elements added to Sn—Zn, and a Sn—Ag based alloy is a Sn—Ag alloy itself or an alloy to which at least one of the above-described third elements is added to Sn—Ag.
Among these lead-free solders, a Sn—Ag based solder composition and particularly a Sn—Ag—Cu based solder composition in which 0.5-1.0% of Cu is added to a Sn—Ag based solder composition is currently most widely used because it is convenient from the standpoint of the melting temperature of the solder and the reliability of heat cycling properties of the solder. The present applicant disclosed a Sn—Ag—Cu based lead-free solder made from an alloy having a composition containing greater than 3.0 and at most 5.0 weight percent of Ag, 0.5-3.0 weight percent of Cu, and a remainder of Sn and which forms soldered portions having excellent thermal fatigue properties, and a Sn—Ag—Cu—Sb based lead-free solder further containing at most 5 mass percent of Sb (JP 5-50286 A, referred to as Patent Document 1). However, in contrast to a conventional Sn—Pb solder, a lead-free solder such as a Sn—Ag—Cu based lead-free solder does not contain Pb which has low reactivity with flux components, so it has high reactivity with flux components. Accordingly, a solder paste made of this lead-free solder needs a special flux for lead-free solders having low reactivity with solder powder.
As electronic equipment becomes smaller, locations for soldering on printed circuit boards are becoming finer, but a space between mounted substrates is becoming smaller, and cleaning of printed circuit boards is becoming difficult. Accordingly, mounting atop printed circuit boards with solder paste is often being carried out using no-clean solder paste instead of water-soluble solder pastes, which always need cleaning.
There are three requirements of solder paste for carrying out mounting on printed circuit boards without cleaning.
First, flux residue after reflow must be non-tacky. If flux residue on a printed circuit board after reflow is tacky, dust or dirt in the air adheres to the flux residue and sometimes causes insulating defects such as leaks.
Second, the color of the flux residue on a printed circuit board after reflow should be light and not prominent. The reason for the second requirement that the flux residue after reflow have a light color and should be close to transparent is that in a final step, soldered portions undergo visual inspection, and if the flux residue has a dark color, erroneous inspection can easily take place and when the printed circuit board is observed by the user at the time of repair or the like, if the color is too dark it presents a poor image.
The third requirement is that flux residue should have good adhesion to the silicone resin or acrylic resin. This is because is that conformal coating is carried out in a printed circuit board for electronic parts with a silicon resin or an acrylic resin,
It is conceivable to use a maleated rosin or a hydrogenated rosin which are rosins as flux of a paste which has flux residue after reflow with a light color which is close to transparent. JP 9-52191 A (referred to as Patent Document 2) discloses a maleated rosin or a hydrogenated rosin as a flux for a solder paste. These prior art documents concern solder paste which uses Sn—Pb solder powder which are all solder powders having low reactivity, and there is no combination with lead-free solder.
Patent Document 1: JP 5-50286 A
Patent Document 2: JP 9-52191 A